1. Field of the Invention
The present invention relates to a process for the continuous mass production of high impact vinylaromatic copolymers.
In more detail, this invention relates to a process for the continuous bulk production of high impact copolymers based on vinylaromatic and monomers.
2. Discussion of the Prior Art
It is well known that the impact resistance properties of polystyrene can be improved by mixing therewith a rubbery material. To obtain this the two components can be mechanically mixed or, in a more favourable way, the styrene polymerization is carried out in the presence of the rubbery material.
In this last case the procedure is generally to dissolve the rubbery material in the monomer styrene and then to polymerize this solution in continuous or discontinuous, in bulk, in solution or in a combined bulk/suspension polymerization process.
Immediately after the beginning of the polymerization reaction the solution of the rubbery material in monomer styrene separates into two phases; one of them, that is a solution of the rubber in monomer styrene, initially forms the continuous phase; while the other, that is a solution of polystyrene resulting in its own monomer, remains dispersed in this phase in drops form. By the increasing of the conversion the quantity of the second phase increases at the expense of the first one; as soon as the quantity of formed polystyrene exceeds the quantity of employed rubber, an exchange takes place in the phase, that is generally called phase inversion.
When this inversion takes place, drops of rubbery material solution are forming in the polystyrene solution; but these drops of rubbery material solution incorporate, on their turn, little drops of what has now become the continuous polystyrenic phase. At the same time, a grafting of the rubber by the polystyrene chains takes place during this polymerization.
Usually the polymerization is carried out in several stages. In the first polymerization stage, that is called prepolymerization, the rubber solution in styrene is polymerized until a conversion is reached that is superior to the phase inversion; afterwards the polymerization is carried on until the desired styrene conversion.
The continuous bulk polymerization is described, for example, in the U.S. Pat. No. 2,694,692, 3,243,481 and 3,658,946. The combined bulk/suspension discontinuous polymerization is described, for example, in the U.S. Pat. No. 3,428,712.
It is well known that the properties of the styrenic polymers, and in particular their stress crack resistance, especially when they come in contact with fatty substances, can be improved by introducing little quantities of a vinylcyanide in the polymer.
It is also known that the improved impact resistance properties of the styrenic polymers can be influenced and varied during the polymerization by varying the grafting degree of the polystyrene chains on the rubber.
Several attempts have been made to this purpose, but none of them has allowed to realize the grafting at the desired degree, in conditions acceptable from an industrial point of view. An attempt to increase the grafting degree of the rubber lies in using block-rubbers of the A-B, A-B-A, or B-A-B type (in which A is a styrene block and B a butadiene block) or in using star-shaped block copolymers. This attempt is described in the patents DE-A-2,646,508, DE-A-2,646,509 DE-A-2,717,777 and DE-A-2,504,118.
However, this attempt is scarcely attractive from an industrial point of view because of the particular operative conditions to be used with those types of rubbers.
It is possible to realize in situ the bond between the rubber and the thermoplastic component, by carrying out the polymerization in the presence of an initiator, usually a conventional peroxide; but in this case problems can be caused by the technical difficulties connected with the polymerization, such as an unsuitable removal of the heat or excessive viscosity of the solution or premature cross-linking of the rubber at high temperatures; only low concentrations can be used. As an alternative, if conventional concentrations of catalyst are employed, only low temperatures can be employed, with consequent low grafting degree and unsufficient mechanical properties.
The U.S. Pat. No. 4,282,334 suggests, in example 1, to increase the grafting degree by carrying out the polymerization, in the initial stage, under adiabatic conditions. Even if this method allows to reach a good grafting degree, the accomplishment of this process on industrial scale involves insuperable difficulties, because the reaction becomes uncontrollable.
The same U.S. patent suggests that a high degree of grafting can be obtained under isothermal conditions in a tubular reactor, in the presence of a catalyst at high temperature. However, tests carried out by the Applicant have demonstrated that also this process is not free from problems, because of the formation of cross-linked polymer, above all on the top of the reactor, so that the reactor is completely obstructed after 1-2 days. These problems are noticed particularly in the case of the copolymerization of styrene with vinyl-cyanide monomer.